Overview on recent W7-AS results

During the almost one year shutdown in 1999/2000 the Stellarator W7-AS underwent two major modifications: First, the limiters were replaced by ten divertor modules, and the diagnostic set associated with the plasma boundary and target plate regions was greatly expanded for divertor experiments. Extensive tests of the island divertor concept foreseen for W7-X were started in 2001. Partial plasma detachment is observed for magnetic field configurations with large edge islands and with high neutral beam heating power at core densities of 3x1020 m-3 . At somewhat lower densities stable discharges with good energy confinement and high radiation levels (mainly edge radiation) can be maintained up to pulse lengths of more than 1 s. Secondly, the previously counter tangential neutral beam injector box was shifted to a co-position. Thus, the heating efficiency is increased at low magnetic fields and high densities, where high-β experiments are usually performed, but where counter beam injection suffers from substantial fast ion losses. In first experiments <β>- values up to about 3% were achieved at still only weak MHD-activity. Meanwhile 5 gyrotrons (70 and 140 GHz) are available on W7-AS with a total heating power of up to 2.5 MW. They were used to increase central electron temperatures up to 6.7 keV. The application of electron Bernstein waves was further investigated for plasma heating and as a plasma diagnostic. Heating of a high density divertor discharge and the measurement of electron temperature profiles are demonstrated. Copyright (2002) Australian National University- Research School of Physical Sciences and Engineering